"We are such stuff as dreams are made of" when it's actually "We are such stuff as dreams are made ON" is one; the above-mentioned line from Hamlet is another pet peeve, also the rampantly willful misunderstanding of Juliet's query: "wherefore art thou Romeo?" to be a question about where he is, rather than about WHY he has to be called that!

Oh---also "bubble bubble, toil & trouble" for "double, double, toil & trouble" in Macbeth---I could (obviously) go on and on.....fer chrissakes, if people are gonna quote Shakespeare, they should at least take the trouble to quote him CORRECTLY!!!!